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N ASA TECHNICAL

MEMORANDUM

APPLICATION OF A GENERAL STRESS, STRAIN-RATE, TEMPERATURE CORRELATION FOR WELDED THICK-WALLED N-155 TUBES UNDERGOING SECONDARY CREEP FROM INTERNAL PRESSURE

by Richnrd E. Morris

Lewis Reseud Center

Clevehlzd, Ohio 44135

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION l WASHINGTON, D. C. l SEPTEMBER 1971

TECH LIBRARY KAFB, NM

1. Report No.

NASA TM x-2372 4. Title and Subtitle

2. Government Accession No.

APPLICATION OF A GENERAL STRESS, STRAIN-RATE, TEM- PERATURE CORRELATION FOR WELDED TRICK-WALLED N- 155 TUBES UNDERGOING SECONDARY CREEP FROM INTERNAL PRESSURE

7. Author(s)

Richard E. Morris

9. Performing Organization Name and Address

Lewis Research Center National Aeronautics and Space Administration Cleveland, Ohio 44 135

2. Sponsoring Agency Name and Address

National Aeronautics and Space Administration Washington, D. C . 2054 6

5. Supplementary Notes

OL5ll970 - 3. Recipient’s Catalog No.

5. Report Date

September 1971 6. Performing Organization Code

8. Performing Organization Report No.

E-6398 10. Work Unit No.

126-15 11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum 14. Sponsoring Agency Code

6. Abstract

A correlation equation relating creep deformation of tubes to a function of stress and temperature is presented. The equation was fitted to experimental N-155 tube test data with a multiple linear regression analysis. A design procedure is proposed and applied to the design of a heat- exchanger tube. Design calculations showed that a welded, thick-walled, N-155 heat-exchanger tube having an outside diameter of 0.635 cm and an inside diameter of 0.391 cm was satisfactory for 10 OOO-hr service life at 1089 K (1500’ F), with 10.34 MN/m2 (1500 psi) internal pressure, and with a limitation of 1 percent on creep deformation during service life.

7. Key Words (Suggested by Author(s) 1

Creep strain-rate correlation Creep in thick-walled tubes Design of thick-walled tubes for creep

9. Security Classif. (of this report) 1 20. Security Classif. (

18. Distribution Statement

Unclassified - unlimited

this page) 21. No. of Pages 22. Price* Unclassified

I- Unclassified 16 1 $3.00

* For sale by the National Technical Information Service, Springfield, Virginia 22151

l-- . ~- A--

APPLICATION OF A GENERAL STRESS, STRAIN-RATE, TEMPERATURE

CORRELATION FOR WELDEDTHICK-WALLED N-155TUBES UNDER-

GOING SECONDARYCREEP FROM INTERNAL PRESSURE

by Richard E. Morris

Lewis Research Center

SUMMARY

Helium-to-air heat-exchanger tubes in the engines of a mobile nuclear propulsion system must operate with high internal pressures at high temperatures. Under these conditions creep will occur continuously during the operating life of the heat exchanger. A method was needed for predicting the creep deformation of tubing as a function of time in operation so that the total creep deformation accumulated during a design lifetime could be limited to some small acceptable value.

A plot of some experimental tube test stress as a function of creep-rate data was available for welded N-155 thick-walled tubes. A procedure was needed for the interpo- lation and the extrapolation of the experimental data for use in design.

This report presents a correlation of the experimental N-155 tube strain rate data as a function of stress and temperature. Material constants in the equation were opti- mized with a multiple linear regression analysis. The correlation equation was investi- gated graphically and found to be representative of the experimental data.

The ranges of data correlated were temperature, 1061 to 1234 K (1450’ to 1760’ F); stress, 16.2 to 77.6 MN/m2 (2.35 to 11. 25 ksi); ratio of tube outside to inside diameter, 1. 154 to 1.623; and test time, 140 to 1857 hr. Experimental strain rates varied from 44. 5x10-6 to 584~10~~ cm/(cm)(hr).

A design procedure is proposed and applied to a design problem. An N-155 tube is designed for use in a heat exchanger. Calculations showed that a thick-walled, welded N-155 heat-exchanger tube could be designed to operate at 1089 K (1500’ F) with a stress of 10.34 MN/m2 (1500 psi) for a 10 OOO-hr lifetime. Total growth in tube diameter is less than 1 percent in 10 000 hr. The actual stress in the tube is less than the 10 OOO-hr creep-rupture stress by a factor of 1.85.

The procedure presented for the analysis of welded, thick-walled, N-155 heat- exchanger tube test data is applicable for the analysis of tube test strain-rate data ob- tained for other tube materials.

INTRODUCTION

High-temperature heat exchangers are required for use in engines for mobile nu- clear propulsion systems. These engines will be operated with hot helium gas at tem- peratures of 1000 to 1150 K, pressurized at 7 to 14 MN/m2.

Under these conditions of operation, the heat-exchanger tubes will creep throughout the operating life of the engine. Loss of coolant through rupture of the tubing must be avoided. The total amount of creep deformation of the tubing at the end of life must be limited in the design of the heat exchanger.

A report on welded N-155 tubing (ref. 1) includes a parametric correlation of stress as a function of the Larson-Miller parameter. This parameter is a function of time and temperature. The strain-rate data were based on diameter measurements taken before and after the tests. Thus, it was assumed that primary creep was negligible and that the strain rate was uniform throughout the lifetimes of the tubes tested. Strain rates were plotted in a graph of stress as a function of strain rate at constant temperature. No method was provided for interpolation or extrapolation of the data for use in design calculations.

One purpose of this report is to provide a systematic procedure for the analysis of strain-rate data for use in the design of thick-walled tubes operating under conditions of internal pressure and high temperature such that creep continues throughout the operat- ing life of the tubes.

Reference 2 provided a system of equations relating the stresses and strain rates in thick-walled tubes undergoing secondary creep from static internal pressure at constant temperature. This report describes an application of equations from the reference re- port. Correlation equations are used to provide creep strain-rate data for welded N-155 tubes which may be used for the accurate interpolation or extrapolation of strain-rate data over a range of temperature and stress in the design of heat-exchanger tubing.

A

a

b

AH

N1

N2

2

SYMBOLS

material constants, hr -’ (Nm-2)-n

inside radius, cm

outside radius, cm

apparent activation energy, J/mole

safety factor, O,/Oa

safety factor, tu/ta

n

P

P

R

S

T

t

X

i

T

P

c

0

stress exponent

Larson-Miller parameter

pressure, NMB2

gas constant, 8.3143 J/(mole)(K)

standard deviation

temperature, K

time, hr

variable

creep rate, hr -1

equivalent creep rate, - fi (; 3 C 8

)2+(; -; )2+(; 8 z Z

-; 2 1’2, hr-’ r )I

b/a stress, Nm -2

equivalent stress , 1 cr - cJ(J2 + (De - Dz)Z + (oz - 2 l/2

or )I , Nmw2 Subscripts:

a

b

C

d

i

m

r

U

Z

e

inside diameter

outside diameter

calculated value of variable

design

experimental value of variable

melting

radical

ultimate

axial

circumferential

ANALYSIS

The empirical strain-rate equation assumed in reference 2 is

n -AH/RT E=A~ e (1)

3

TABLE I. - CHEMICAL COMPOSITION OF N-155a SPECIMENS

F Heat Sample Carbon Manganese Silicon Phosphorous Sulfur Chromium Nickel Molybdenum Cobalt Columbium Tungsten Nitrogen or tantalum Specimen size Diameter Wall thickness

Composition, wt %

cm in. cm

+ _____ ---__

in.

C-183(

AMS 5585 0.08 to 1.00 to

I I

bl.OO

0.16 2.00

bo. 030 bo. 030 20.0 to 19.00 to

22.50 21.00

0.953

0.635

0.025

0.048

As- 0.14 j 1.65 1 0.60 0.015 0.003

.016 .009

21.02 20.04

21.04 19.91

I T i- 0.250 0.122 0.013 1 0.006 21.27 119.62 3.00 / 19.66 1 0.98

L 24 after 10 1. 63 .49 ,015 ,010 ~ 20.54 19.75 ( 2.97 19.63 1.12 test 1 I - %-on base alloy.

bMaximum.

TABLE II. - WELDED N-155 TUBE DATA

Specimen Temperature,

size a K

A 1144 47.4 A 1144 47.4 A 1152 34.5 A 1152 34.5 A 1152 34.5 A 1152 34.5 A 1061 77.6 A 1061 77.6 A 1061 77.6 A 1222 19.4 A 1153 34.5 A 1153 34.5 A 1153 47.4 A 1153 47.4 A 1214 20.3 A 1214 20.3 A 1214 30.9 A 1214 30.9 B 1178 24.7 B 1178 24.7 B 1178 24.7 B 1178 24.7 B 1214 16. 2 B 1214 16. 2 B 1214 16.2 B 1234 16.2 B 1234 16. 2 B 1234 24.7 B 1234 24.7

Equivalent

stress in

bore,

MN/m2

-Equivalent

strain rate

in b

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